Proton ATPases are a large class of membrane-proteins that use the energy of ATP hydrolysis to generate an electrochemical proton gradient across a membrane. The resultant gradient may be used to transport other ions across the membrane (Na.sup.+, K.sup.+, or Cl.sup.-), or to maintain an acidic environment which is important to the function of many cellular vesicles (Mellman, I. et al. (1986) Ann. Rev. Biochem. 55:663-700). Proton ATPases are further subdivided into the mitochondrial F.sub.1 F.sub.0 ATPases, the plasma membrane ATPases, and the vacuolar ATPases.
The vacuolar proton ATPases (vp-ATPases) function in eukaryotic cells to establish and maintain an acidic pH within various vesicles involved in the processes of endocytosis and exocytosis. Such vesicles include phagosomes, lysosomes, endosomes, and secretory vesicles. Endocytosis is the process in cells of internalizing nutrients, solutes or small particles (pinocytosis) or large particles such as internalized receptors, viruses, bacteria, or bacterial toxins (phagocytosis). Exocytosis is the process of transporting molecules to the cell surface. It facilitates placement or localization of membrane-bound receptors or other membrane proteins and secretion of hormones, neurotransmitters, digestive enzymes, wastes, etc. Endocytosis and exocytosis are fundamental processes to the function of all types of cells.
Eukaryotic vp-ATPase is a multimeric enzyme composed of 3-10 different subunits. One of these subunits is a highly hydrophobic polypeptide of approximately 16 kDa that is similar to the proteolipid component of vp-ATPases from eubacteria, fungi, and plant vacuoles (Mandel, M. et al. (1988) Proc. Natl. Acad. Sci. 85:5521-24). The 16 kDa proteolipid component is the major subunit of the membrane portion of vp-ATPase and functions in the transport of protons across the membrane.
The proteolipid subunit from bovine chromafin granule membranes is 155 amino acids in length and is characterized by the presence of four potential transmembrane segments, one of which contains a glutamic acid residue that is a potential binding site for N,N'-dicyclohexylcarbodiimide (DCCD). DCCD is a specific inhibitor of ATPase activity in both vacuolar and mitochondrial proton ATPases. Tyrosine residues at positions 68 and 144 may also play a role in the protonation-deprotonation process (Mandel et al., supra).
More recently, a proteolipid-like protein, PPA1, that is homologous to the bovine chromaffin proteolipid subunit has been identified in yeast (Apperson, M. et al. (1990) Biochem. Biophys. Res. Comm. 168(2):574-79). PPA1 differs from the bovine proteolipid primarily by the presence of a 46-amino acid N-terminal sequence not found in the bovine molecule. PPA1 is important for cell growth in yeast and like the bovine proteolipid, may be a component of a proton pump. The novel N-terminal amino acid sequence may target PPA1 to a different cell compartment. Alternatively, PPA1 may be involved in the transport of substrates such as sugars, vitamins, or ions in addition to, or instead of, protons. A third PPA1-like proteolipid has been found in Caenorhabditis elegans (Wilson, R. et al., (1994) Nature 368:32-8).
The discovery of a new proton ATPase subunit, and polynucleotides encoding it satisfy a need in the art by providing new compositions which are useful for the diagnosis, prevention, and treatment of cancer, immune disorders, neurological disorders and developmental disorders.